1. Field of the Invention
The present invention relates to the control technique for a torque transmission mechanism, particularly the control technique for a torque transmission mechanism including a synchronizer member to synchronize a first rotational shaft and a second rotational shaft.
2. Description of the Background Art
A vehicle incorporating a transfer capable of switching between a two-wheel drive mode and a four-wheel drive mode is conventionally known. Some of such vehicles are further capable of switching between a high speed running mode and a low speed running mode in the four-wheel drive mode. When the drive mode is switched, the shafts differing in revolution speed are set to the same revolution speed. In other words, torque is required to set the shafts differing in revolution speed to the same revolution speed when the drive mode is to be switched. The torque required for switching the drive mode varies by the friction torque of the transmission coupled to, for example, the transfer. There is proposed the technique of controlling the actuator of the transfer such that torque corresponding to the friction torque of the transmission is output.
Japanese Patent Laying-Open No. 2004-322702 discloses a drive mode switching control apparatus for a transfer, including a transfer disposed at a position between an automatic transmission and a wheel, having a high gear/low gear switching mechanism that allows switching between a high mode position by a high gear ratio and a low mode position by a low gear ratio, and a drive mode switching control unit providing a switch actuation command of the drive mode position to a shift actuator when a predetermined switching condition including the condition that the automatic transmission takes the neutral range position is met at the time of switching the drive mode in response to an operation by the driver. This drive mode switching control apparatus includes a friction torque estimation unit to estimate the friction torque when the automatic transmission is at the neutral range position. The drive mode switching control unit sets the switch actuation torque designated to the shift actuator by the variable torque corresponding to the estimated friction torque at the time of switching between the high mode position and low mode position.
The drive mode switching control apparatus disclosed in the aforementioned publication can ensure a reliable switching operation and improve the fuel efficiency by reducing the energy consumption at the shift actuator, independent of the level of the friction torque of the automatic transmission, by virtue of setting the switch actuation torque designated to the shift actuator in accordance with the variable torque corresponding to the estimated friction torque.
In the case where the actuator is controlled such that torque corresponding to the friction torque is output as in the drive mode switching control apparatus disclosed in Japanese Patent Laying-Open No. 2004-322702, the actuator is controlled such that more torque is output in proportion to higher friction torque. In this case, the pressure acting on the synchronizer that synchronizes the rear output shaft and the front output shaft, for example, of the transfer through the frictional force is increased. This may accelerate the wear of the synchronizer and the like.